The present invention relates to an acoustic structure including one or two Helmholtz resonators.
Among the conventionally-known acoustic structures including a Helmholtz resonator, such as bass reflex type speakers and resonance type sound absorbing panels, are ones which can set the Helmholtz resonator at a desired resonant frequency. Japanese Patent Application Laid-open Publication No. HEI-04-159898 (hereinafter referred to as “patent literature 1”) and Japanese Patent Application Laid-open Publication No. 2005-86590 (hereinafter referred to as “patent literature 2”), for example, disclose a technique for setting a resonant frequency by adjusting a length L of a neck (or neck length L) from among three factors that determine a resonant frequency of a Helmholtz resonator, i.e. an area S of an open surface (open surface area S) of a neck, a volume V of a cavity communicating with the neck, and the neck length L from a boundary surface between the neck and the cavity to the open surface of the neck.
In the bass reflex type speaker disclosed in patent literature 1, a bass reflex port of a cylindrical shape is fixed at its open end to a front wall portion of a speaker enclosure. Within the speaker enclosure, there are provided a cylindrical auxiliary port that surrounds the outer periphery of the bass reflex port, and a drive mechanism for driving the auxiliary port to move along the outer periphery of the bass reflex port. Further, in this bass reflex type speaker, the bass reflex port and the auxiliary port function as the neck of the Helmholtz resonator.
As well known in the art, there exists predetermined relationship among the area S of the open surface of the neck, volume V of the cavity, neck length L and resonant frequency f in a Helmholtz resonator as shown in the following mathematical expression:f=(c/2π)[S/{(L+ΔL)V}]1/2  (1),where “c” indicates sound speed, and “ΔL” indicates an open end correction value (if the radius of the open surface is indicated by r, then ΔL=0.85r×2).
Thus, it is possible to increase or raise the resonant frequency f of the bass reflex type speaker disclosed in patent literature 1 by moving the auxiliary port toward the front surface (i.e., by decreasing the neck length L) and decrease or lower the resonant frequency f by moving the auxiliary port away from the rear surface (i.e., by increasing the neck length L). Therefore, a user of this bass reflex type speaker can set a lower limit frequency of a sound enhancing frequency band through driving of the auxiliary port.
The sound absorbing device disclosed in patent literature 2 includes top and bottom surface plates opposed to each other via four side surface plates, and an accordion-shaped hose having an open end provided in the top surface plate and extending toward the bottom surface plate. In this sound absorbing device, the accordion-shaped hose functions as the neck of the Helmholtz resonator. The resonant frequency f of the sound absorbing device disclosed in patent literature 2 is increased (or raised) by contraction of the hose and decreased (or lowered) by expansion of the hose. Thus, a user of the sound absorbing device can set a frequency of a sound to be absorbed, through contraction/expansion of the hose.
With the techniques disclosed in patent literatures 1 and 2 above, however, there would be presented the problem that it is almost impossible to vary the resonant frequency unless the cylindrical member functioning as the neck is designed to be capable of being expanded sufficiently.